Method of making a lead on chip (LOC) semiconductor device

ABSTRACT

A semiconductor device including a lead on chip structure employs two frames. One of the frames includes a die pad and an outer frame portion and the other frame includes a plurality of leads and an outer lead portion. After a semiconductor chip is die bonded to the die pad, the two frames are connected to each other with the leads extending across the semiconductor chip. Slits within the second frame provide access to parts of the outer frame of the first frame and the first frame is severed at those slits. The severed portions of the first frame are removed after which the leads of the second frame are connected by wire bonding to the semiconductor chip. Finally, the semiconductor chip, the remaining part of the first frame, and the second frame are encapsulated in a resin with leads extending from the resin. The remaining parts of the outer frame of the second frame are removed by cutting and the exposed leads outside the resin are formed into a desired shape.

This disclosure is a division of application Ser. No. 08/070,990, filedJun. 4, 1993, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device havinglead-on-chip structure with leads extending over a surface of asemiconductor chip, and relates to a method for producing it. Thepresent invention also relates to a leadframe for use in such a deviceor in production of such a device.

2. Description of the Related Art

FIG. 35 is a cross sectional view of a conventional semiconductor devicehaving lead-on-chip structure (hereafter referred to as LOC-structure).Such a device is disclosed, for example, in Japanese Patent ApplicationLaid-Open No.2-45969. As shown in FIG. 35, the device comprises a diepad 1, a semiconductor chip 2, and a plurality of leads 3 extendingtoward the semiconductor chip from its both sides, each of the pluralityof leads 3 comprising an inner lead portion 3a and an outer lead portion3b. The device further comprises thin metal wires 5, molding resin 6,and a plurality of electrodes 4 formed along the both sides on a primarysurface of the semiconductor chip 2.

An integrated circuit (not shown) and electrodes 4 are formed byphotolithography or the like on the upper primary surface of thesemiconductor ship 2. The semiconductor chip 2 is fixed on the die pad 1by bonding the back primary surface of the semiconductor chip 2 to thedie pad 1 with a conductive adhesive such as a conductive resin. Theinner lead portions 3a are each electrically connected to the electrodes4 on the semiconductor chip 2 via the thin metal wires 5. These elementsdescribed above are sealed with molding resin 6 except for the outerlead portions 3b so that each outer lead portion 3b of leads 3 isexposed to the outside. The outer lead portions 3b of the leads 3 areeach formed into a desired shape such as a straight type, gull wingtype, J-type, etc.

The Japanese Patent Application Laid-Open No.2-45969 also discloses amethod for producing such a device described above. In accordance withthis method, two frames are used: a first frame comprising an outerframe and a die pad disposed inside the outer frame, the die pad beingconnected to the outer frame via a suspending lead; and a second framecomprising an outer frame and a plurality of leads extending inward fromthe outer frame. The die pad of the first frame is sunk (depressed)downward by an amount larger than the thickness of the semiconductorchip. After a semiconductor chip is die-bonded on the die pad of thefirst frame, the second frame is connected onto the first frame so thateach lead extends over the semiconductor chip with a predeterminedconstant space between each lead and the upper surface of the chip.Then, wire-bonding and resin-molding are performed. The unnecessaryportions of frames such as outer frames are removed to obtain aseparated semiconductor device. Finally, forming is performed for eachouter lead portion of the leads, thus a completed semiconductor deviceis obtained.

In a conventional semiconductor device with LOC-structure describedabove, a semiconductor chip is die-bonded onto a die pad with aconductive resin or the like. However, resin materials exhibit moistureabsorption, thus moisture absorbed in resin may cause degradation in theadhesive strength, and/or may corrode the semiconductor chip or leads incontact with the resin. To mount a semiconductor device on a circuitboard or the like, the semiconductor device is put on the circuit board,then the circuit board with the chip are placed in a hot environment toheat it. Thus, the outer lead portions are soldered to the circuitboard. The moisture absorbed in the die-bonding resin confined in themolding resin is evaporated during the soldering process, which mayresult in separation of the semiconductor chip from the die pad, and/orresult in cracks in a package.

In the conventional production method using two frames described above,the outer frames of two frames remain connected to each other until themolding process is completed. As a result, these outer frames act asobstacles, and the difficulty occurs in handling. Therefore, during thesucceeding processes after the two frames are connected, failures oftenoccur in transferring frames, and/or liquid used for processingpenetrates between the outer frames, leaking later to causecontamination in environments. In particular, in the case of aproduction process including a step for plating the outer lead portionsof the semiconductor device before separating the semiconductor devicefrom the frames, serious problems occur because plating solution canpenetrate between two outer frames and may leak some time later.

In another method known in the art, single frame is used to produce asemiconductor device having LOC-structure, the single -frame comprisinga die pad and leads formed in an integral fashion. In such a type ofleadframe, a die pad is disposed between the leads extending from bothsides of the leadframe, and the die pad extends in the directionsperpendicular to these leads. However, the width of the die pad cannotbe enlarged exceeding the lead area. As a result, only narrow die padsare available. Japanese Patent Application Laid-Open No.64-69041discloses a leadframe having a die pad with partially large widthextending beyond the area of leads. In this case, however, the length ofleads in turn, should be shortened, or otherwise, the leads should bedeformed. As a result, longer thin metal wires are required forwire-bonding and/or locations of electrodes on the semiconductor chipare limited.

In the case where a semiconductor device is produced using a leadframehaving a die pad as well as leads formed in an integral fashion, the diepad is sunk by the amount corresponding to the thickness of asemiconductor chip, and the semiconductor chip is inserted between theleads and the die pad, then the semiconductor chip is die-bonded on thedie pad. However, in the case where the die pad extends perpendicular tothe direction in which the leadframe is transferred during theproduction processes of the semiconductor device, the semiconductor chipmust be inserted between the leads and the sunk die pad in the samedirection as that of the leadframe-transfer path. This insertion processis difficult to perform, and complicated and troublesome operations arerequired. Furthermore, it also requires complicated and troublesomeoperations to die-bond a semiconductor chip onto the die pad with hardsolder after the semiconductor chip is inserted. There are such problemsin conventional semiconductor devices and methods for Producing them.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve the problems describedabove. More specifically, it is an object of the present invention toprovide a higher-reliability semiconductor device having LOC-structurein which no corrosion occurs. It is another object of the presentinvention to provide a method for producing such a semiconductor devicewithout liquid-leakage problem during production processes. It is afurther object of the present invention to provide a leadframe for usein such a semiconductor device.

To achieve the above objects, in accordance with a first aspect of thepresent invention, a semiconductor device having LOC-structure isprovided wherein a semiconductor chip is die-bonded onto a die pad witha hard solder material exhibiting no moisture absorption.

In accordance with a second aspect of the present invention, a method isprovided for producing such a semiconductor device by using two frames,that is, a frame for leads and a frame for die pad, wherein immediatelyafter these two frames are connected to each other, an outer frameportion of the frame for die pad is cut off and removed viaframe-cutting slits formed in the frame for leads.

A third aspect of the present invention provides a leadframe comprisingtwo frames including a frame for leads, having frame-cutting slits, foruse in production of a semiconductor device in accordance with thesecond aspect of the invention.

Furthermore, a fourth aspect of the present invention provides aleadframe for use in production of a semiconductor device, comprising adie pad and leads formed in an integral fashion, the die pad including amain pad disposed between two sets of leads, the one of sets of leadsbeing formed on one side of an outer frame of the leadframe, and theother one of sets of leads being formed on the other side of the outerframe of the leadframe, the main pad extending perpendicular to theseleads extending inward from both sides of the outer frame of theleadframe, the die pad further including at least one branch padextending approximately perpendicular to the main pad and extendingbetween neighboring two leads extending from the above outer frame.

A fifth aspect of the present invention provides a method for producinga semiconductor device by using a leadframe in accordance with thefourth aspect of the present invention. In this method, the width of adie pad is widened so as to ensure that a semiconductor chip can belocated on the die pad in a more stable manner. Another feature of thismethod is that when a semiconductor chip is heated via a die pad so asto make wire-bonding easier, the efficiency of heat conduction can beimproved.

A sixth aspect of the present invention provides a leadframe including adie pad extending inward from an outer frame of the leadframe, theleadframe further including leads extending toward the die pad from bothsides of the outer frame of the leadframe, the die pad and the leadsbeing formed in an integral fashion. In this leadframe, the die pad isconnected to the portions of the outer frame on both sides, wherein boththe portions are bent in the same direction perpendicular to theleadframe plane so as to form U-shaped portions, so that the die pad issunk from the outer frame and also from the leads. The U-shaped portionshave a size large enough to insert a semiconductor chip through one ofthese U-shaped portions.

A seventh aspect of the present invention provides a method forproducing a semiconductor device by using a leadframe in accordance withthe sixth aspect of the present invention, the leadframe including a diepad extending in the direction perpendicular to the leadframe-transferdirection, the leadframe further including leads extending toward thedie pad from both sides of the outer frame of the leadframe, the die padand the leads being formed in an integral fashion. In this method, theportions of both sides of the outer frame, at which the die pad isconnected to the outer frame, are bent in the same directionperpendicular to the frame plane so as to form U-shaped portions havinga size large enough to insert a semiconductor chip through one of theseU-shaped portions. Then, a semiconductor chip is inserted through one ofthe U-shaped portions formed in both sides of the outer frame, theU-shaped portions being located at the sides of the leadframe-transferpath.

An eighth aspect of the present invention provides a leadframe includinga die pad and leads formed in an integral fashion within an outer frame,wherein only one end portion of the die pad is connected to the outerframe so that the die pad can be bent outward from the outer frame, andso that a semiconductor chip can be die-bonded onto the die pad bentoutward.

A ninth aspect of the present invention provides a method for producinga semiconductor device by using a leadframe in accordance with theeighth aspect of the present invention. In this method, after bending adie pad outward from an outer frame, a semiconductor chip is die-bondedonto the die pad bent outward. Then, the die pad with the semiconductorchip on it is bent back to the position at which the leads extendingover the semiconductor chip have a predetermined constant space betweenthe leads and the primary surface of the semiconductor chip.

A tenth aspect of the present invention provides a leadframe including adie pad and leads formed in an integral fashion in such a way that thedie pad is formed in a region of the frame and the leads are formed inother region of the frame adjacent to the first region of the frame. Thedie pad is connected to the frame via suspending leads. One of thesesuspending leads is cut, then the other suspending lead is bent so thatthe die pad is put over the leads.

An eleventh aspect of the present invention provides a method forproducing a semiconductor device by using a leadframe in accordance withthe tenth aspect of the present invention. In this method, afterdie-bonding a semiconductor chip onto a die pad, unnecessary portions ofa frame around the die pad are cut off, then a suspending lead extendingtoward the lead area is bent into a U-shape so that the die pad with thesemiconductor chip is put over the lead area with a predeterminedconstant space between the die pad and the extending leads.

A twelfth aspect of the present invention provides a leadframe modifiedfrom a leadframe of the tenth aspect of the present invention in such away that the suspending lead to be cut is made longer. In a process,this suspending lead is cut in such a way that the suspending leadremains connected to the die pad. After the die pad is folded over theleads, the long suspending lead remaining connected to the die pad isbent toward the leads so as to make connection between the end portionof this remaining suspending lead and the frame of the lead area. Thus,more firm connection of the die pad can be achieved.

A thirteenth aspect of the present invention provides a method forproducing a semiconductor device by using a leadframe in accordance withthe twelfth aspect of the present invention. In accordance with thismethod, as described above, a die pad can be fixed more firmly to aframe of a lead area.

A fourteenth aspect of the present invention provides a die-bondingmethod comprising steps of: putting and pressing metal foil on a diepad; putting a semiconductor chip on the metal foil; heating these fromthe surroundings so as to melt the metal foil; stopping heating so as tosolidify the melted metal foil so that the semiconductor chip is fixedto the die pad.

In the first aspect of the present invention, as described above, hardsolder exhibiting no moisture absorption is used as a die-bondingmaterial for die-bonding a semiconductor chip. As a result, moisture isnot absorbed into the die-bonding material. Thus, it is possible toavoid corrosion of the semiconductor chip, and to avoid cracks in apackage.

In the method for producing a semiconductor device in accordance withthe second aspect of the present invention, and in the leadframe for usein the production of the semiconductor device in accordance with thethird aspect of the present invention, after two frames are connected toeach other, unnecessary portions of the frame for die pad is cut off andremoved via frame-cutting slits. As a result, during succeedingprocesses after that, the leadframe can be treated as if it comprisesonly one sheet of frame. As a result, production processes can besimplified and can be performed easily. In particular, leakage ofplating solution can be effectively avoided.

In the leadframe and the method for producing a semiconductor deviceusing this leadframe, in accordance with the fourth and fifth aspects ofthe present invention, respectively, the leadframe comprises a die padand leads formed in an integral fashion, the die pad including a mainpad and branch pads extending approximately perpendicular to the mainpad, wherein the width and the area of the die pad are made larger. Eachof the branch pads extends between adjacent two leads among a pluralityof the leads extending inward from an outer frames, thus the existenceof the branch pads causes no limitation in length and shape of theleads. Furthermore, because the width and the area of the die pad areenlarged, a semiconductor chip can be located on the die pad in a morestable manner. Moreover, improvement of the efficiency of heatconduction between the die pad and the semiconductor chip can beachieved and wire-bonding can be performed more easily.

In the sixth and seventh aspects of the present invention, regarding aleadframe and a method for producing a semiconductor device using thisleadframe, respectively, a leadframe is used which includes a die padextending in the direction perpendicular to the direction in which theleadframe is transferred during the processes for producing asemiconductor device. A semiconductor chip can be inserted between thedie pad and the leads passing through one of U-shaped portions formed onboth sides of the outer frame, the U-shaped portions being located onthe sides of leadframe-transfer path. Thus, the chip-insertion processis simplified and can be performed easily. In addition, in a leadframein accordance with the sixth aspect of the present invention,independently of the direction in which the die pad extends, the die padcan be sunk from the leads by bending the portions of both sides of theouter frame, at which the die pad is connected to the outer frames, intoa U-shape. This makes the leads extending from one side closer to theleads extending from the other side. Thus, it is possible to make theleads reach the locations closer to the central part of thesemiconductor chip.

In the eighth and ninth aspects of the present invention, regarding aleadframe and a method for producing a semiconductor device using thisleadframe wherein the leadframe includes a die pad and leads formed inan integral fashion, the die pad can be bent outward from the outerframe. In this situation in which the die pad is bent outward, asemiconductor chip can be die-bonded onto this die pad. This allowssimpler and easier die-bonding process, compared to the case where asemiconductor chip is inserted between a die pad and leads.

In the tenth and eleventh aspects of the present invention, regarding aleadframe and a method for producing a semiconductor device using thisleadframe wherein the leadframe includes a die pad and leads formed inan integral fashion, a semiconductor chip can be easily die-bonded tothe die pad without any additional special process.

In the twelfth and thirteenth aspects of the present invention,regarding a leadframe and a method for producing a semiconductor deviceusing this leadframe, the die pad can be connected to a frame for leadsmore firmly by modifying the above leadframe and method in accordancewith the tenth and eleventh aspects of the invention in such a way thata suspending lead portion remaining connected to the die pad is bent andis further connected to the frame for leads. Thus, the succeedingprocesses after that can be performed more easily and more accurately.

In the die-bonding method in accordance with the fourteenth aspect ofthe present invention, metal foil such as solder disposed between a diepad and a semiconductor chip is heated from the surroundings so as tomelt the metal foil. Thus, this method allows easier die-bondingprocess, and it provides a significant advantage in particular for thecase where diebonding is performed after a semiconductor chip isinserted between the die pad and inner lead portions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view of one embodiment of a semiconductordevice in accordance with a first aspect of the present invention;

FIG. 1B is a side view, partially cross sectioned, of a semiconductordevice of FIG. 1A;

FIG. 2 is a perspective view showing the structure of a leadframerelating to a second and third aspects of the present invention;

FIGS. 3A-3C are plan views of a semiconductor device for explanation ofone embodiment of successive production steps in accordance with asecond aspect of the present invention;

FIGS. 4A-4D are plan views of a semiconductor device for explanation offurther successive production steps following the step in FIG. 3C;

FIG. 5 is a flowchart showing a method for producing a semiconductordevice of FIGS. 3A-3C and FIGS. 4A-4D;

FIG. 6 is a side view for explanation of a first embodiment of a methodfor connecting two frames in accordance with the second aspect of thepresent invention;

FIG. 7 is a side view for explanation of a second embodiment of a methodfor connecting two frames in accordance with the second aspect of thepresent invention;

FIG. 8 is a side view for explanation of a third embodiment of a methodfor connecting two frames in accordance with the second aspect of thepresent invention;

FIGS. 9A and 9B are perspective views for explanation of a fourthembodiment of a method for connecting two frames in accordance with thesecond aspect of the present invention;

FIGS. 10A-10D are perspective views for explanation of a fifthembodiment of a method for connecting two frames in accordance with thesecond aspect of the present invention;

FIG. 11 is a plan view of two frames for use in the second aspect of thepresent invention, the two frames being in the situation in which twoframe are connected to each other;

FIGS. 12A and 12B are side views for explanation of one example ofmethods for cutting a frame with regard to a production method inaccordance with the second aspect of the present invention;

FIG. 13 is a plan view showing one embodiment of a leadframe relating toa fourth and fifth aspects of the present invention;

FIG. 14 is a side view of the leadframe of FIG. 13, seen in thedirection of the arrow XIV;

FIG. 15 is a cross sectional view showing one embodiment of asemiconductor device produced by a production method in accordance withthe fifth aspect of the present invention;

FIG. 16 is a cross section of the semiconductor device of FIG. 15, takenalong the lines XVI--XVI;

FIG. 17 is a plan view showing another embodiment of a leadframerelating to the fourth and fifth aspects of the present invention;

FIG. 18 is a perspective view showing further another embodiment of aleadframe in accordance with the fourth aspect of the present invention;

FIG. 19 is a plan view showing one embodiment of a leadframe relating toa sixth and seventh aspects of the present invention;

FIG. 20 is a flowchart showing a method for producing a semiconductordevice in accordance with the seventh aspect of the present invention;

FIG. 21 is a perspective view showing one embodiment of a leadframerelating to an eighth and ninth aspects of the present invention;

FIG. 22 is a perspective view showing the situation of a die-pad bendingstep by a method for producing a semiconductor device in accordance withthe ninth aspect of the present invention;

FIG. 23 is a perspective view showing the situation of a die-bondingstep by a method for producing a semiconductor device in accordance withthe ninth aspect of the present invention;

FIG. 24 is a side view showing the situation of a die-pad bending-backstep and a wire-bonding step by a method for producing a semiconductordevice in accordance with the ninth aspect of the present invention;

FIG. 25 is a perspective view showing another example of a die-padbending step by a method for producing a semiconductor device inaccordance with the ninth aspect of the present invention;

FIG. 26 is a perspective view showing one embodiment of a leadframerelating to a tenth and eleventh aspects of the present invention;

FIG. 27 is a perspective view showing the situation of a die-bondingstep by a method for producing a semiconductor device in accordance withthe eleventh aspect of the present invention;

FIG. 28 is a side view showing the situation of a die-pad folding stepand a wire-bonding step by a method for producing a semiconductor devicein accordance with the eleventh aspect of the present invention;

FIG. 29 is a perspective view showing the situation of a die-bondingstep by a method for producing a semiconductor device relating to atwelfth and thirteenth aspects of the present invention;

FIG. 30 is a side view showing the situation of a die-pad folding step,die-pad fixing step, and wire-bonding step by a method for producing asemiconductor device in accordance with the thirteenth aspect of thepresent invention;

FIG. 31 is a schematic view showing one embodiment of a die-bonding by amethod for producing a semiconductor device in accordance with afourteenth aspect of the present invention;

FIG. 32 is a flowchart showing one example of methods for producing asemiconductor device in accordance with the ninth aspect of the presentinvention;

FIG. 33 is a flowchart showing examples of methods for producing asemiconductor device in accordance with the eleventh and thirteenthaspects of the present invention;

FIG. 34 is a flowchart showing one example of diebonding methods inaccordance with the fourteenth aspect of the present invention; and

FIG. 35 is a cross sectional view of a conventional semiconductor deviceof a similar type.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, referring to the accompanying drawings, the preferred embodimentswill be described below.

THE FIRST ASPECT OF THE INVENTION

FIGS. 1A and 1B show an embodiment of a semiconductor device havingLOC-structure in accordance with the first aspect of the presentinvention, wherein FIG. 1A is a perspective plan view showing innerstructure and FIG. 1B is a side view, partially cross sectioned, of thedevice. As shown in these figures, the device comprises a die pad 1, asemiconductor chip 2, leads 3 having an inner lead portion 3a and anouter lead portion 3b, electrodes 4, thin metal wires 5, and moldingresin 6 to form a package. This device has substantially the samestructure as that of conventional semiconductor devices except that theelectrodes 4 are formed along a line in a central part of thesemiconductor chip.

There are also provided common leads 3c primarily used for supplyingelectrical power or used as a ground lead, wherein the common leads 3cextend over the central portion of the semiconductor chip 2 in thelongitudinal direction. In addition, hard solder 7 such as usual solderis also provided for diebonding the semiconductor chip 2 to the diepad 1. The hard solder 7 exhibits no moisture absorption, thus there isno possibility that moisture corrodes the semiconductor chip 2.Furthermore, there is no possibility that moisture brings about cracksin a package, even if the whole device is heated when a completedsemiconductor device is attached on a circuit board (not shown) bysoldering the outer lead portions 3b. Because the hard solder material 7for connecting the semiconductor chip 2 to the die pad is disposed in aninner portion covered with molding resin 6, less amount of heat isprovided to this hard solder material 7 compared to the externalportions. Thus, the hard solder material 7 does not melt duringsoldering process. If necessary, the hard solder material 7 may beselected so that it has a melting point high enough not to be meltedduring soldering process for mounting the semiconductor device on acircuit board. In the case of usual solder, this can be achieved byselecting the mixture ratio between tin and lead.

THE SECOND AND THIRD ASPECTS OF THE INVENTION

FIG. 2 shows a leadframe comprising two frames in accordance with thesecond and third aspects of the present invention. The second aspect ofthe present invention relates to a method for producing a semiconductorchip using a leadframe, and the third one relates to the leadframeitself. As shown in FIG. 2, a leadframe 80 comprises a frame 81 forleads and a frame 82 for die pad for use in the present invention. Theseframes 81 and 82 are each produced from one metal sheet by means ofcutting such as punching or etching. The frame 81 for leads comprisescommon leads 3c and a plurality of leads 3 extending inward from bothsides of the outer frame 81a. Each of the leads 3 comprises an innerlead portion 3a and an outer lead portion 3b. The frame 81 for leads hasframe-cutting slits 18 formed in the outer frame 81a. The frame-cuttingslits 18 will be described later in more detail. The frame 82 for diepad includes a die pad 1, both sides of the die pad 1 being connected toinner portions of an outer frame 82a via suspending leads 82b. The diepad 1 is formed in such a shape similar to the shape of a semiconductorchip 2 to be die-bonded so that the semiconductor chip 2 can be fixedonto the die pad in a more stable manner. The semiconductor chip 2 isdie-bonded onto the die pad 1 via a hard solder material 7, as shown inFIG. 2. In the frame 82 for die pad, the suspending leads on the bothsides are bent by the amount determined by taking the thickness of thesemiconductor chip 2 into account, so that the die pad 1 is sunk fromthe surrounding outer frame 82a. Thus, when the frames 81 and 82 areconnected to each other, the common leads 3c and the inner lead portion3a of each lead 3 extend over in parallel to the primary surface of thesemiconductor chip 2, keeping a predetermined constant space between theleads and the surface of the semiconductor chip 2 The method ofconnection between the frame 81 for leads and the frame 82 for die padwill be described later.

In one of known technologies, a die pad is formed in an integral fashionbetween common leads 3c or between inner lead portions 3a extending fromthe both sides of an outer frame 81a of a frame 81 for leads. However,in such a frame, the width of the die pad 1 cannot be widened beyond theregion of leads. In contrast, in this specific aspect of the presentinvention, wherein two frames, that is, a frame 81 for leads and a frame82 for die pad, are connected to each other, the shape of die pad 1 canbe designed independently of the shape of leads, thus the width of thedie pad 1 can be widened sufficiently as required. A semiconductor chip2 may be also die-bonded in a different way such that after two frames81 and 82 are connected to each other, the semiconductor chip 2 isinserted between the die pad 1 and the inner lead portions 3a, andfurther semiconductor chip 2 is fixed onto the die pad 1.

A method for producing a semiconductor device in accordance with thesecond aspect of the present invention is illustrated in FIGS. 3A-3C andFIGS. 4A-4D. A flowchart of this production method is also shown in FIG.5. Now, the successive steps of this method will be described next.First, a semiconductor chip 2 is fixed onto a die pad 1 of a frame 82for die pad shown in FIG. 3A, by using a hard solder material 7 such asusual solder exhibiting no moisture absorption (See FIGS. 1 and 2). Now,the situation becomes as shown in FIG. 3B (die-bonding step S1). Then, aframe 81 for leads having frame-cutting slits 18 as shown in FIG. 3C isput on the frame 82 for die pad which is in the situation as shown inFIG. 3B, and the frames are connected to each other as shown in FIG. 4A,by one of methods (for example, spot welding) which will be describedlater (connecting step S2).

Then, an unnecessary outer frame 82a of the frame 82 for die pad is cutoff by means of for example die-cutting. Thus, the die pad 1 with thesemiconductor chip 2 remains connected to connecting portions 17 on bothsides (outer-frame cutting-off step S3). After that, the resultantcombined frames can be treated as a sheet of leadframe, and thesuccessive production processes can be performed easily. Moreover, thisresults in elimination of the possibility of accumulation or leakage ofplating solution used in later process of exterior-plating. Then,electrodes 4 (see FIG. 1) formed along the center line of thesemiconductor chip 2 are electrically connected via the thin metal wires5 to the inner lead portions 3a and/or the common leads 3c extendingover the chip 2 (wire-bonding step S4).

After wire-bonding is performed, the frame is put into a mold (notshown), then resin is injected into it so as to seal the main padincluding the semiconductor chip 2 with molding resin 6 as shown in FIG.4D (molding step S5). In this step, molding is performed in such a waythat the outer lead portions 3b are not covered with the resin but areexposed to the outside. The following steps are not shown in figures,but the outer lead portions 3b are exterior-plated (exterior-platingstep S6), then the semiconductor device including the outer leadportions 3b are cut off from the frame and tie-bars are also cut off soas to separate the outer lead portions 3b into individual ones.Furthermore, the separated outer lead portions 3b are formed into adesired shape in lead forming step S7, thus a completed semiconductordevice is obtained.

The frame-cutting slits 18 may be formed in any one of two frames. Theshape of the slits is not limit to that shown in figures. Instead ofdie-cutting, laser cutting or the like may be used for the same purposefor cutting off the outer-frame portion of the frame 82 for die pad. Inthis specific example in accordance with the first aspect of the presentinvention, the semiconductor chip 2 is die-bonded with a hard soldermaterial 7, however other materials may be used not only in this firstaspect but also in any of the other aspects of the present invention.

Now, in accordance with the second aspect of the present invention,there will be described some examples of the methods for connecting aframe 81 for leads to a frame 82 for die pad. FIG. 6 shows a firstembodiment of a method for connecting a frame 81 for leads to a frame 82for die pad. In this embodiment shown in FIG. 6, connection is performedby means of resistance welding which is one of spot-welding techniques.In this figure, there are shown welding electrodes 9, anelectrode-holding bar 10a for holding the upper welding electrode 9, andan electrode-holding base 10b for holding the lower welding electrode 9.A frame 81 for leads and a frame 82 for die pad are put between theupper and lower welding electrodes and pressed by them by moving theelectrode-holding bar 10a in the direction denoted by the arrow A inFIG. 6. By supplying a current between the upper and lower weldingelectrodes 9, the frames 81 and 82 are spot-welded. Resistance weldinghas advantages that large bonding strength can be obtained and that onlylittle deformation occurs at the surface of a welded portion. Anothergreat advantage of resistance welding is that welding dust is notgenerated, thus extremely little contamination occurs to thesemiconductor chip 2. Spot welding can be also performed by means oflaser welding.

FIG. 7 shows a second embodiment of a method for connecting a frame 81for leads to a frame 82 for die pad. In this embodiment shown in FIG. 7,connection is performed by using an adhesive tape. In this figure, thereare shown an adhesive tape 11, a pressing fixture 12, a frame-holdingbase 13. An adhesive tape 11 is placed between frame 81 for leads and aframe 82 for die pad, and these frames 81 and 82 with the adhesive tape11 are put on the frame-holding base 13 and are pressed by the pressingfixture 12 so that both frames 81 and 82 are connected to each other viathe adhesive tape 11. This method has an advantage over a laser weldingtechnique that it can offer high throughput and also has an advantageover a spot welding technique that there is no need for maintenance ofequipment.

FIG. 8 shows a third embodiment of a method for connecting a frame 81for leads to a frame 82 for die pad. In this embodiment shown in FIG. 8,connection is also performed by using an adhesive tape in a similar wayto that of FIG. 7, however, in this case, the adhesive tape 11 comprisesan adhesive tape base 11a and thermoplastic adhesives 11b coated on itsboth surfaces. In this embodiment, each surface of the adhesive tape 11is bonded to the frame 81 or frame 82 via an interfacial material ofthermoplastic adhesive 11b. As a result, even if thermal history isintroduced during processing steps such as wire-bonding and molding(sealing), the thermoplastic adhesives 11b absorb the stress introducedby the difference in thermal expansion between both frames 81 and 82,thus the total thermal deformation of the frames can be minimized.Although an adhesive tape is used in these connecting methods shown inFIGS. 7 and 8, the connection is performed at the portions outside ofthe area to be molded with molding resin 6. Thus, even if the adhesivetape exhibits moisture absorption, no problem will occur.

FIGS. 9A and 9B show a fourth embodiment of a method for connecting aframe 81 for leads to a frame 82 for die pad. In this embodiment,connection is performed by using a rivet. In these figures, there areshown holes 14 formed in each of both frames 81 and 82, and a rivet 15.As shown in FIG. 9A, two frames 81 and 82 are put one on top of theother, then a rivet 15 is inserted through holes 14. Then, the upper endof the rivet 15 is pressed and spread out so as to fasten two fittedframes 81 and 82. The shapes of the rivet 15 and holes 14 are notlimited to those shown in figures. Instead of the rivet 15, any otherelement having similar function may be used.

FIGS. 10A-10D show a fifth embodiment of a method for connecting a frame81 for leads to a frame 82 for die pad. In this embodiment, connectionis performed by means of calking. In these figures, there are shown aprojection 16 formed in a frame 82, a hole 14 formed in a frame 81,wherein the projection 6 is to be fitted into the hole 14. First, aprojection is formed in a frame 82 as shown in FIG. 10A by means ofetching or punching. Then, the projection 16 is bent upward as shown inFIG. 10B. A hole 14 corresponding to the projection 16 is formed in aframe 81 by means of etching or punching. One of two frames 81 and 82 isput on the other in such a way that the projection 16 is inserted andfitted in the hole 14. Then, the projection 16 is bent back as shown inFIG. 10D. Thus, the frames 81 and 82 are fitted and fastened by means ofcalking. The shapes of the projection 16 and the hole 14, and the methodfor forming them are not limited to those described above.

FIGS. 11, 12A and 12B show one embodiment of a method for cutting off anunnecessary portion of an outer frame 82a of a frame 82 for die padwhich has been fastened to a frame 81 for leads by means of any methoddescribed above. FIG. 11 shows a frame 82 for die pad with asemiconductor chip mounted on a die pad, the frame 82 for die pad beingconnected to the bottom of a frame 81 for leads. In this figure, thereare shown connecting portions 17 (for example, spot welding portions) atwhich frames are connected to each other, and frame-cutting slits 18formed in the frame 81 for leads.

The frame-cutting slits 18 formed in the frame 81 for leads as describedabove offer a means for cutting off the frame 82 for die pad except fora die pad 2 (including suspending leads) connected to the frame 81 atconnecting portions 17 on both sides. As a result of the cutting, theframe 81 having the die pad 1 with the semiconductor chip 2 can betreated almost like a one-sheet frame. Thus, during the succeedingprocesses, problems such as miss-transfer of frames, and accumulationand leakage of solution which often occur in combined two frames can beeffectively avoided.

This cutting method is shown in FIGS. 12A and 12B where a lower metalfixture is denoted by 19, a frame holder by 20, and a cutting metalelement by 21. As shown in FIG. 12A, the frame 81 for leads and die padframe 82 which are connected to each other at the connecting portion 17are put on the lower metal fixture 19 and held by the frame holder 20.The cutting metal element 21 is inserted into the frame-cutting slits 18down to a position at which the cutting metal element 21 stops With apredetermined adequate gap to the lower metal fixture 19. Thus, theouter frame-of the frame 82 for die pad is cut off from the frame 81 forleads.

As described previously, one known method for producing a semiconductordevice having LOC-structure is to use a one-sheet leadframe having leadsand a die pad formed in an integral fashion. Such a frame can beproduced from a sheet of metal by means of cutting such as punching oretching. Although this method has an advantage that less processingsteps are required for producing a semiconductor device compared to thecase where two frames are combined, it has disadvantages that because ofits structure it is difficult to expand the width of a die pad exceedinga lead area, and that a semiconductor chip cannot be die-bonded on a diepad in a stable fashion. Now, to solve such problems, in accordance withthe fourth aspect of the present invention, a leadframe for use in asemiconductor device having LOC-structure as well as a method inaccordance with the fifth aspect of the present invention for producinga semiconductor device having LOC-structure using such a leadframe willbe described below.

THE FOURTH AND FIFTH ASPECTS OF THE INVENTION

FIGS. 13 and 14 show an example of a leadframe in accordance with thefourth and fifth aspects of the present invention, wherein FIG. 13 is atop view and FIG. 14 is a side view seen in the direction denoted by anarrow XIV in FIG. 13. As shown in FIG. 13, this leadframe 83 comprises aplurality of leads 3 inside of an outer frame 83a and also comprises adie pad 100 extending in the direction approximately perpendicular tothe leads 3, the leads 3 and the die pad 100 being formed in anintegrated fashion. The die pad 100 comprises a conventional type ofmain pad 101 extending along the area corresponding to a central area ofa semiconductor chip 2 in a range of its full length, and furthercomprises branch pads 102 extending from both sides of the main pad 101in a cross shape. Thus, the width of the die pad is substantiallywidened. Each branch pad 102 is formed on the same plane on which themain pad 101 exists, and extends approximately perpendicular to the mainpad 101. Furthermore, each branch pad 102 extends between neighboringleads among a plurality of leads 3 extending inward from both sides ofthe outer frame 83a at a predetermined interval. Because of such astructure, the die pad 100 can be widened exceeding the lead areawithout modifying the location, shape, and/or length of leads 3. Asshown in FIG. 14, the die pad 100 is sunk from the outer frame 83a, anda semiconductor chip 2 is die-bonded onto the sunk die pad 100 with forexample hard solder.

FIGS. 15 and 16 show a semiconductor device produced by a productionmethod in accordance with the fifth aspect of the present invention,using a leadframe shown in FIG. 13. FIG. 15 is a cross sectional viewtaken along a lead 3 of the semiconductor device. FIG. 16 is a crosssectional view taken in the line XVI--XVI. The basic structure is thesame as that of a semiconductor device in accordance with the firstaspect of the present invention shown FIG. 1, thus the same or similarportions are denoted by the same numerals. Explanation will not berepeated again for these same or similar portions. In the semiconductordevice shown in FIGS. 15 and 16, there are provided only leads 3extending over the semiconductor chip 2 and there are no common leads 3.The flowchart showing this production method will be the same as that inaccordance with the second aspect of the present invention shown in FIG.5 except that steps S2 and S3 are deleted.

Now, the method for producing a semiconductor device shown in FIGS. 15and 16 will be described. As described above in connection with FIG. 14,a semiconductor chip 2 is inserted between the outer frame 83a of theleadframe 83 and the sunk die pad 100. Then, the semiconductor chip 2 isattached onto the die pad 100 with for example hard solder 7 (diebonding step S1). As shown in FIG. 15, each of leads 3 floats withoutcontact with the upper surface of the semiconductor chip 2.

Then, inner lead portions 3a of the leads 3 extending over thesemiconductor chip 2 are each connected to electrodes 4 on thesemiconductor chip 2 with thin metal wires 5 by means ofultrasonic-thermocompression wire-bonding (wire bonding step S4). Inthis step, when a thin metal wire 5 is compressed onto an electrode 4 onthe semiconductor chip 2, mechanical force is applied to thesemiconductor chip 2. However, due to the good stability of thesemiconductor chip 2 die-bonded on the die pad 100, it is ensured thatwire bonding can be performed easily with no problems. During thewire-bonding process, the die pad is heated so as to raise thetemperature of the semiconductor chip 2 for easier wire bonding. Becausethe die pad 100 has a larger area compared to one in a conventionalleadframe having leads and a die pad formed in an integrated fashion,better thermal conduction can be achieved from the die pad 100 to thesemiconductor chip 2. Thus, the temperature of the semiconductor chip 2can be raised more effectively.

Then, the semiconductor chip 2, the die pad 100, the inner lead portions3a, and thin metal wires 5 are encapsulated with for example epoxy resininto one body by means of for example transfer molding, thus moldingresin 6 is obtained (molding step S5). Each outer lead portion 3bextending outward from the molding resin 6 is exterior-plated (exteriorplating step S6). Finally, an assembled semiconductor device isseparated from the outer frame 83a of the leadframe 83, then each outerlead portion 3b extending outward from the molding resin 6 of thesemiconductor device is formed into a desired shape (lead forming stepS7). Thus, a completed semiconductor device is obtained. A leadframe 83used in actual production comprises a plurality of unit elements suchone shown in FIG. 13, the unit elements being successively connected toeach other, and a plurality of semiconductor devices are produced at atime in a similar way to that of the second aspect of the presentinvention. If unnecessary, exterior plating may not be performed notonly in this specific production method but in any production methodswhich will be described later.

In the completed semiconductor device as in FIGS. 15 and 16, unlikesemiconductor devices produced by using a conventional one-sheet frame,the back surface of the semiconductor chip 2 has a less area in directcontact with the molding resin 6, thus better adhesion is achievedbetween the back surface of the semiconductor chip 2 and the moldingresin 6, and separation is avoided.

In the above specific embodiment, the die pad 100 has a shape of cross.However, the branch pad 102 may be formed only on one side of the mainpad 101 of the die pad 100. In contrast to this, in a leadframe 83 shownin FIG. 17, each side of a main pad 101 of a die pad 100 has a pluralityof branch pads 102 extending from each side. Each branch pad 102 of thisleadframe 83 extends between neighboring leads 3. By using such aleadframe 83 having a die pad 100 with a larger number of branch pads102, the stability of a semiconductor chip and thermal conductionefficiency during a wire bonding process step can be more improved.Furthermore, a completed semiconductor device exhibits better adhesionbetween the back surface of a semiconductor chip and molding resin.

As described above in connection with FIGS. 13 and 17, in a leadframe inaccordance with the fourth aspect of the present invention, a die pad100 extends in the direction perpendicular to the longitudinal directionof a leadframe 83 (actual leadframe comprises a plurality of unitelements shown in these figures, the unit elements being successivelyconnected in the horizontal direction of these figures). Therefore, toinsert a semiconductor chip 2 between a die pad 100 and an outer frame83a, the chip 2 is inserted in the longitudinal direction of theleadframe 83 (the direction denoted by the arrow XIV in FIG. 13). Inproduction lines, in general, leadframes are transferred in thelongitudinal direction of the leadframes. This means that there isdifficulty in chip-insertion process due to the fact that semiconductorchips are inserted in the same direction as that of theleadframe-transfer path in the case of the leadframe 83 shown in FIGS.13 and 17.

In view of the above, another embodiment is provided in accordance withthe fourth aspect of the present invention as is shown in FIG. 18. Inthe case of a leadframe 83 shown in FIG. 18, a main pad 101 of a die pad100 extends in longitudinal direction of the leadframe 83, that is, inthe same direction as that in which the leadframe 83 is transferred, andeach lead 3 extends perpendicular to this direction. Both end portionsof the die pad 100 are bent such that the die pad 100 is sunk from theouter frame 83a. Thus, a semiconductor chip can be inserted from a sideposition of the leadframe-transfer path, that is, in the directionperpendicular to the leadframe-transfer path, as shown by the arrow B.In this way, easier chip-insertion process is achieved. In FIG. 18,there are shown tie-bars 3d for connecting leads 3 to each other, whichare not shown in figures for previous embodiments. These tie-bars 3d areto be cut off and separated into individual outer leads 3b in leadcutting and forming process in which the semiconductor device is alsoseparated from the outer frame 83a.

THE SIXTH AND SEVENTH ASPECTS OF THE INVENTION

In accordance with the sixth and seventh aspects of the presentinvention, FIG. 19 shows a leadframe which is modified from theleadframe shown in FIG. 13 or 17 so that a semiconductor chip can beinserted from a side position of a leadframe-transfer path. The sixthaspect of the invention relates to a leadframe and the seventh to amethod for producing a semiconductor device using a leadframe of thesixth aspect of the invention. In previous embodiments, a die pad 100 issunk from an outer frame 83a by bending both end portions (suspendingleads) of the die pad 100, as shown in FIG. 14. In contrast, in aleadframe 84 shown in FIG. 19, both side portions of an outer frame 84ato which a die pad 100 is connected are bent into U-shape (U-shapedportions 84b) so that the die pad 100 is sunk from the outer frame 83a.Such a leadframe having a die pad 100 sunk by bending portions of theouter frame 84a into U-shape has following advantages: (1) The die pad100 can be sunk more deeply; (2) This technology can be adopted to evena thin leadframe having small mechanical strength; and (3) Leads 3extending from both sides can be located close to the leads on theopposite side. These advantages can be obtained independently of therelation in direction between the die pad and the frame-transfer path.Thus, a leadframe in accordance with the sixth aspect of the presentinvention has no limitations in a direction in which a die pad extends,and it can be used in various fashions.

FIG. 20 is a flowchart showing a production method in accordance withthe seventh aspect of the present invention.

Now, an example of a production method in accordance with the seventhaspect of the present invention will be described. First, by punching oretching a sheet of flat metal (not shown) in a similar way to those inthe embodiments described above, a leadframe 84 is produced in a flatform which is the situation prior to that shown in FIG. 19 (leadframeproduction step S1). A die pad 100 is formed in such a way that its mainpad 101 extends perpendicular to the direction in which the leadframe 84is to be transferred during production steps of a semiconductor device(that is, perpendicular to the longitudinal direction of a leadframe, inthis case). Then, the portions of both sides of outer frame 84a to whichthe die pad is connected (that is, the portions of outer frame locatedat both sides of leadframe-transfer path) are bent by means of forexample pressing so as to form U-shaped portions 84b. These U-shapedportions are projected in the same direction approximately perpendicularto the frame plane. In this way, the die pad 100 is sunk from the outerframe 84a and inner lead portions 3a (die-pad depression step S2). Asemiconductor chip (not shown) is inserted in the direction denoted byan arrow B between the die pad 100 and the inner lead portions 3a viaone of these U-shaped portions 84b (semiconductor chip insertion stepS3). Therefore, the U-shaped portions 84b should have a large enoughsize for a semiconductor chip to be inserted via one of them. After thisstep, a semiconductor device is completed through the steps similar tothose in the case of previous embodiments such as die bonding step S4,wire bonding step S5, molding step S6, exterior-plating step S7, andlead forming step S8.

In accordance with this aspect of the present invention, leadframes 84are transferred in the longitudinal direction of the leadframes 84during production processes and semiconductor chips are inserted from aside of the transfer path. Thus, the chip-insertion process can beperformed easily. Furthermore, because both sides of the outer frame 84aare bent into the U-shape, it is possible to shorten the distance fromthe inner lead portions 3a extending from one side of the outer frameand those extending from the other side. As a result, the inner leadportions 3a can extend to more inner locations of the semiconductorchip.

THE EIGHTH AND NINTH ASPECTS OF THE INVENTION

The present invention further includes another method for producing asemiconductor device using a sheet of frame comprising a die pad andleads formed in an integrated fashion.

In accordance with eighth and ninth aspects of the present invention, afirst embodiment is shown in FIGS. 21-24 relating to a leadframe and amethod for producing a semiconductor device using this leadframe. Inthese figures, there are shown a leadframe 85, an outer frame 85a, a diepad 100, and a plurality of leads 3. The die pad 100 comprises a mainpad 101 and branch pads 102. Each lead 3 comprises an inner lead portion3a and an outer lead portion 3b. There is also provided tie-bars 3d forconnecting leads to each other. This leadframe is the same as that shownin FIG. 13 in accordance with the fourth aspect of the present inventionexcept that only one end portion of the die pad 100 is connected to theouter frame 85a. FIG. 32 is a flowchart showing a production method inaccordance with the ninth aspect of the invention.

Referring to the figures, one embodiment of a production method inaccordance with the ninth aspect of the invention will be describedbelow. First, for example one flat sheet of metal (not shown) is cut bypunching or etching so as to produce a leadframe 85 as in FIG. 21. Here,one end of a die pad 100 is connected to an outer frame 85a, and aplurality of leads 3 extend from both sides of the outer frame 85atoward the die pad 100. Then, the die pad 100 is bent outward from theouter frame 85a as shown in FIG. 22 (die pad bending step S1). In thisstep, the die pad 100 is bent such that the die pad 100 is approximatelyperpendicular (at 90 degrees) to the outer frame 85a. Then, as shown inFIG. 23, a semiconductor chip 2 is attached onto the bent die pad 100with hard solder 7 such as common solder (die bonding step S2). In thisway, a semiconductor chip can be easily die-bonded onto a die pad 100.

Now, as shown in FIG. 24, the die pad 100, having the semiconductor chip2 mounted on it, is bent back toward the outer frame 85a (die-padbending-back step S3). As shown, the die pad 100 is bent in the form ofL-shape so that the inner lead portion 3a of each lead 3 extends over aprimary surface of the semiconductor chip 2 having electrodes 4 and acircuit (not shown) formed on the primary surface of the semiconductorchip 2 in such a way that a predetermined constant space is maintainedbetween the primary surface and the inner lead portions 3a. Then, therespective electrodes 4 on the semiconductor chip 4 are connected to thecorresponding inner lead portions 3a of leads 3 with thin metal wires 5by means of ultrasonic-thermocompression wire bonding, thus electricalconnections are achieved (wire bonding step S4). After this step, asemiconductor device is completed through the steps similar to those inthe case of previous embodiments such as molding step S5,exterior-plating step S6, and lead forming step S7. In the abovespecific embodiment, a semiconductor chip 2 is die-bonded onto the diepad 100 after the die pad 100 is bent by 90 degrees with respect to theouter frame 85a. However the bending angle of the die pad 100 is notlimited to that. The die pad 100 may be bent by 180 degrees as shown inFIG. 25, or by any arbitrary degree which results in easy die bonding.

THE TENTH AND ELEVENTH ASPECTS OF THE INVENTION

In accordance with tenth and eleventh aspects of the present invention,a first embodiment is shown in FIGS. 26-28 relating to a leadframe and amethod for producing a semiconductor device using this leadframe. Inthese figures, there is shown a wide leadframe 86 comprising an outerframe 86a, a die pad 110 in the sheet form, and leads 3. The outer frame86a includes three bone frames. The die pad 110 has a size similar tothat of a semiconductor chip 2 to be mounted on it. Both ends of the diepad 110 are connected respectively to a first and second bone frames ofthe outer frame 86a via suspending leads 111. The leads 3 are formedbetween the second and third bone frames. A production method inaccordance with the eleventh aspect of the present invention is shown ina flowchart of FIG. 33.

Referring to the figures, a production method in accordance with theeleventh aspect of the present invention will be described below. First,as shown in FIG. 26, for example one flat sheet of metal (not shown) iscut by punching or etching so as to produce a wide leadframe 86 having adie pad and leads formed in an integrated fashion. In this leadframe 86,leads 3 are formed in a half-unit frame area and a die pad is formed inanother neighboring half-unit frame area. Because the leadframe 86 isformed in such a way described above, the die pad 110 can be formed inthe shape of a sheet which makes possible to directly wire-bond a chiponto the die pad without any further modification. As shown in FIG. 27,a semiconductor chip 2 is attached onto the die pad 110 with hard solder7 such as common solder (die bonding step S1). Then, the outer frame 86ais cut at three half-etched portions 86b so as to remove an unnecessaryportion (lower portion including the first bone frame in FIG. 27) of theouter frame 86a around the die pad 110 (outer-frame cutting step S2).Furthermore, the die pad 110 with the mounted semiconductor chip 2 isbent at the suspending lead 111 into the U-shape so that the die pad 110is folded toward the half-unit frame area of the outer frame 86a inwhich the leads 3 are formed, as shown in a cross sectional view of FIG.28 (die-pad folding step S3). In this way, the inner lead portion 3a ofeach lead 3 is arranged with a predetermined constant space over aprimary surface (upper surface) of the semiconductor chip 2 whereinelectrodes 4 and a circuit (not shown) are formed on the primarysurface. Then, a semiconductor device is completed through the stepssimilar to those in the case of previous embodiments, such as a wirebonding step S4, molding step S5, exterior-plating step S6, and leadforming step S7. Japanese Patent Application Laid-Open No.63-34966discloses a technology in which a leadframe is produced in the form of asheet having a die pad and leads formed in an integrated fashion, and anarea of the outer frame of the leadframe is folded so as to put leadsover the die pad in a similar way to that of the present aspect of theinvention. However, there is significant difference that in the presentaspect of the invention an unnecessary portion of an outer frame arounda die pad is cut off, then a suspending lead 111 for connecting the diepad to the outer frame is bent into the U-shape so as to put the die padover the leads. As can be seen, not only the above process steps can beeasily performed, but also in succeeding steps, the leadframe can beeasily handled. Moreover, because the unnecessary portion is removed,less miss-transfer occurs during the succeeding process steps.

THE TWELFTH AND THIRTEENTH ASPECTS OF THE INVENTION

FIGS. 29 and 30 show a leadframe and a method for producing asemiconductor device using this leadframe in accordance with twelfth andthirteenth aspects of the present invention. The differences from thecase of a leadframe described above in connection with FIG. 27 are thatone of suspending leads on both sides of a die pad, that is, asuspending lead 112 extending in the direction opposite to leads 3, andbeing connected to a first bone frame is made longer than in the case ofthe previous embodiment and that there is provided a thinner half-etchedportion 113 at the end of the suspending lead 112, and that there isprovided a fixing means, that is, positioning hole 86c in a third boneframe located at the opposite portion to the die pad 110 in an area offrame 86a in which the leads 3 are formed. The process steps of thisembodiment can be described by a flowchart added with a step denoted bydotted lines in FIG. 33, wherein FIG. 33 is a flowchart showing theeleventh aspect of the present invention.

In this embodiment, in a process step for cutting off an unnecessaryportion of the outer frame 86a around the die pad 110 as well as thehalf-etched portions 86b of the outer frame 86a, the half-etched portion113 is cut so that the longer suspending lead 112 may remain connectedto the die pad 110. Then, by bending a suspending lead 111 into theU-shape as shown in FIG. 30, the die pad 110 with a mountedsemiconductor chip 2 is folded toward the frame area 86a in which theleads 3 are formed (die-pad folding step S3). After that, the longersuspending lead 112 is bent into the L-shape and the thinned half-etchedportion 113 is inserted into (or connected to) the positioning hole 86cformed in the third bone frame of the outer frame 86a so as to fixingthe die pad 110 to the leads 3 (die-pad fixing step S8). Thus, thepresent aspect of the invention provides a way adaptable to the massproduction, which ensures that the distance between the surface of thesemiconductor chip 2 and the inner lead portions 3a can be kept at acertain constant value, which further results in better productionyields. The other process steps are the same as those in the case of theeleventh aspect of the present invention.

THE FOURTEENTH ASPECT OF THE INVENTION

In accordance with the fourteenth aspect of the present invention, therewill be described a preferred embodiment of a die-bonding method whichcan be used in the die-bonding steps of the above-mentioned methods forproducing the semiconductor devices. FIG. 31 shows an example ofdie-bonding methods used in production of semiconductor device, inaccordance with the fourteenth aspect of the present invention. FIG. 34is a flowchart showing this die-bonding method. In actual production, asdescribed earlier, in order to produce a plurality of semiconductordevices at a time, a leadframe, for example a leadframe 85 shown inFIGS. 21 and 22, comprises a plurality of unit elements successivelyconnected to a neighboring unit element, wherein each unit elementcomprises a die pad 100 and leads 3. The process steps described beloware successively performed for one unit element at a time by shiftingthe leadframe 85 by one block (one pitch). In the first step, metal foilcomprising for example hard solder 7 such as usual solder is put andpressed onto a die pad 100 (metal foil putting and pressing step S1).Then, the leadframe 85 is shifted by one pitch and a semiconductor chip2 is put on the metal foil 7 on the die pad 100 (semiconductor chipputting step S2). After another one-pitch shifting, the die pad 100 isheated via its bottom portion by non-contacting heating means such asradiation from a heating lamp 50 so as to melt the metal foil 7 (metalfoil melting step S3). By stopping heating (or by intentionallycooling), the melted metal foil 7 is solidified and the semiconductorchip 2 is connected onto the die pad 100.

In actual production lines, in accordance with the die-bonding methoddescribed above, die bonding can be easily performed by shifting aleadframe by one pitch at a time. Especially in the case where asemiconductor chip is inserted between leads of a leadframe and a sunkdie pad, this method makes die-bonding process significantly easier. Inthe specific embodiment described above, metal foil 7 is melted byheating it via the bottom of a die pad so that a circuit formed on theupper surface of a semiconductor chip may not be damaged. However, ifthe degree of heating is under a permissible level, the heating can beperformed from the upper side or from both upper and lower sides.

As described above, the present invention provides a various advantages.

In a semiconductor device in accordance with the first aspect of thepresent invention, hard solder exhibiting no moisture absorption is usedas a die-bonding material for die-bonding a semiconductor chip. As aresult, moisture is not absorbed into the die-bonding material, and itis possible to avoid the problems such as corrosion of the semiconductorchip, cracks in a package, and separation between the semiconductor chipand the die pad. Thus, a higher-reliability semiconductor device can beobtained.

In a method in accordance with the second aspect of the presentinvention for producing a semiconductor device, after two leadframes,that is, one for leads and the other for a die pad, are connected toeach other, an unnecessary portion of an outer frame of the leadframefor a die pad is cut off and removed from the other portions viaframe-cutting slits formed in the frame for leads. Thus, during thesucceeding processes after that, the leadframes can be treated as if theleadframe comprises only one frame. As a result, production processescan be simplified and can be performed easily. Therefore, the productionefficiency and production yield can be improved. The third aspect of thepresent invention provides a leadframe having an advantage that aftertwo leadframes, one for leads and the other for a die pad, are connectedto each other, an unnecessary portion of one frame can be cut off andremoved from the other portions.

In accordance with the fourth aspect of the present invention, aleadframe comprises a die pad including branch pads formed on both sidesof the main die-pad portion, wherein each branch pad extendsperpendicular to the main die-pad portion, and wherein each branch padextends between leads. In this way, the width and the area of the diepad can be made substantially larger without modifying leads. The fifthaspect of the present invention provides a production method which makespossible to hold and fix a semiconductor chip on a die pad in a morestable manner, and further to improve thermal conduction efficiencybetween the die pad and the semiconductor chip. As a result, productionprocesses especially a wire-bonding process can be performed easily,accurately, and efficiently. Thus, a high-reliability semiconductordevice can be produced. Compared to a semiconductor device produced byusing a conventional leadframe having a narrow die pad, thesemiconductor device in accordance with the present aspect of theinvention has less area at which the semiconductor chip is in directcontact with molding resin. As a result, there is less possibility ofseparation of the molding resin from the semiconductor chip. Thus, ahigher-reliability semiconductor device can be obtained.

In a leadframe in accordance with the sixth aspect of the presentinvention, the portions on both sides of an outer frame to which a diepad is connected are bent into the U-shape so as to sink the die pad.Compared to the case where the die pad is sunk by bending the suspendingleads on both sides of the die pad, this leadframe has advantages thatthe die pad can be sunk more deeply, and that larger mechanical strengthcan be obtained, and that it is possible to shorten the distance betweenleads extending from one side and those extending from the other side.The seventh aspect of the present invention provides a production methodwhereby a die pad extending perpendicular to the direction in which theleadframe is transferred during the processes of producing asemiconductor device is sunk by bending both side portions of the outerframe to which the die pad is connected into the U-shape, so that asemiconductor chip can be inserted from a location at the side of theleadframe-transfer path. Thus, the chip-insertion process can beperformed easily. Furthermore, when the side portions are bent into theU-shape so as to form the U-shaped portions, the die pad is sunk fromthe leads, and, at the same time, the leads extending from one sidebecome closer to the leads extending from the other side. As a result,it is possible to make the leads reach the positions closer to thecentral part of the semiconductor chip, and it is possible to increasethe flexibility in the electrode location.

In accordance with the eighth and ninth aspects of the present inventionrelating to a leadframe and a method for producing a semiconductordevice using this leadframe wherein the leadframe includes a die pad andleads formed in an integral fashion, the die pad can be bent outwardfrom the outer frame, and in this situation where the die pad is bent, asemiconductor chip can be die-bonded onto this die pad. This allowseasier and more reliable process, compared to the case where asemiconductor chip is inserted between a die pad and leads.

In accordance with the tenth and eleventh aspects of the presentinvention relating to a leadframe and a method for producing asemiconductor device using this leadframe wherein the leadframe includesa die pad and leads formed in an integral fashion, the die pad is formedwithin an outer frame area adjacent to another outer frame area in whichleads are formed, both sides of the die pad being connected to the outerframe via suspending leads. A semiconductor chip is die-bonded onto thedie pad, then an unnecessary portion of the outer frame around the diepad is cut off. After that, the suspending lead connected to the side ofthe die pad closer to the lead area is bent into the U-shape such thatthe die pad having the mounted semiconductor chip is folded over theleads. This process can be performed easily. Furthermore, because theunnecessary portion of the outer frame is removed, various advantagesare obtained such that the succeeding process steps are made easier, andthat miss-transfer during the production processes can be decreased andthe production efficiency can be improved.

In accordance with the twelfth and thirteenth aspects of the presentinvention with regard to a leadframe and a method for producing asemiconductor device using this leadframe, more firm connection of thedie pad to an outer frame area for leads is achieved by modifying theleadframe and the method in accordance with the tenth and eleventhaspects of the invention in such a way that a suspending lead portionremaining connected to the die pad is bent and connected to the outerframe area for leads. Thus, the succeeding processes can be performedmore easily and more accurately.

In accordance with the fourteenth aspect of the present invention withregard to a die-bonding method, metal foil is put and pressed onto a diepad, then a semiconductor chip is put on this metal foil and the metalfoil is melted by heating it from the surroundings. Then, the heating isstopped so as to solidify the metal foil and to attach the semiconductorchip onto the die pad. This process step is carried out by shifting theleadframe by one pitch at a time. Thus, this method is suitable for usein a production line to die-bond a chip on a leadframe which istransferred on the production line. This method provides significantadvantages that die-bonding process becomes easier and more effectiveand that the production efficiency can be improved, in particular forthe case where die-bonding is performed after a semiconductor chip isinserted between the die pad and inner lead portions.

What is claimed is:
 1. A method for producing a semiconductor devicehaving a lead on chip (LOC) structure using a first frame including anouter frame in a first plane and a die pad connected to the outer frameand displaced from the first plane, and a second frame comprising anouter lead frame, a plurality of leads extending inwardly from the outerlead frame, and frame-cutting slits in said outer lead frame of saidsecond frame for cutting a portion of said first frame after said secondframe is connected to said first frame, said method comprising,sequentially:die-bonding a semiconductor chip to said die pad of saidfirst frame; connecting said second frame to said first frame with aninner lead portion of each of said leads extending across saidsemiconductor chip mounted on said die pad and with the slits in saidsecond frame exposing parts of said outer frame of said first frame;cutting said exposed parts of said outer frame of said first frame atthe frame-cutting slits of said second frame and removing said exposedparts of said outer frame of said first frame, leaving a remaining partof said first frame including said die pad connected to said secondframe; wire-bonding wires between said semiconductor chip and said innerlead portions of said leads; encapsulating said semiconductor chip, saidremaining part of said first frame including said die pad, said wires,and parts of said second frame in a resin body with an outer leadportion of each of said leads exposed outside of said resin body;plating said outer lead portions exposed outside of said resin body;cutting said outer lead frame of said second frame exposed outside ofsaid resin body to separate said outer lead portions from each other;and deforming each of said outer lead portions exposed outside of saidresin body into a desired shape.
 2. The method of claim 1 includingconnecting said second frame to said first frame by resistance welding.3. The method of claim 1 including connecting said second frame to saidfirst frame by applying an adhesive tape between said first and secondframes and pressing said first and second frames against each other. 4.The method of claim 3 wherein said adhesive tape includes an adhesive ontwo opposite sides of said adhesive tape.
 5. The method of claim 1including connecting said second frame to said first frame by riveting.6. The method of claim 1 including producing a projection extending fromone of said first and second frames and forming a hole for receiving theprojection in the other of said first and second frames and connectingsaid second frame to said first frame by inserting the projection intothe hole and bending the projection to hold said first and second framestogether.